The hashtag #Dadbod went viral on social media a few months ago as a lighthearted reference to guys with a "nice balance" of beer gut and buff. But for real dads - especially new and recent ones - the demise in physique after fatherhood is no joke.

Since becoming a first-time father a little more than two years ago, Bernard Soh has put on 8kg and now has a body mass index of 29.4 that's on the cusp of severe obesity. Once a fit and trim recreational cyclist, he has completely fallen off the saddle.

"Not enough sleep; wake-ups in the middle of the night; not enough time to do things, with added chores. Basically, no time to exercise," says Soh, 44, reeling off a list of effects fatherhood has had on his health. "I think it's the same for mums and dads, except we don't get to breastfeed."

Weight gain is common in men after having their first child, according to a recent study by Northwestern University that tracked the weight of more than 10,000 men for up to 20 years from adolescence to young adulthood.

Over the course of the study, the typical 182-cm man who lived with his child gained an average of about 2kg after becoming a first-time dad, or about 1.5kg for those who didn't live with the child. In contrast, childless men lost 600 grams.

Controlling for other variables such as age, race, education, income, daily activity, screen time and marriage status, the weight gain translates to a 2.6 per cent rise in BMI for resident dads and a 2 per cent rise in BMI for non-resident dads.

"Fatherhood can affect the health of young men above the already known effect of marriage," says Dr Craig Garfield, associate professor of paediatrics and of medical social sciences at Northwestern, and lead author of the study in the American Journal of Men's Health. "The more weight the fathers gain and the higher their BMI, the greater risk they have for developing heart disease as well as diabetes and cancer."

Fatherhood has also been linked with a higher prostate cancer risk. In a 2008 study in the journal Cancer, researchers at Copenhagen's Statens Serum Institut analysed data from all men born in Denmark between 1935 and 1988, and found that men without children were 16 per cent less likely than fathers to be diagnosed with prostate cancer during up to 35 years of follow-up.

Paradoxically, among fathers, the more children one has, the lower the risk of disease. The authors suggested that, theoretically, this might reflect a "healthy father" phenomenon, in which men who retain fertility are less likely to develop a malignancy.

Being a first-time father also affects the mind. Mental health experts emphasise the dangers of post-natal depression for mothers, but University of Kansas researcher Carrie Wendel-Hummell says fathers should also pay attention to their mental health around the time of childbirth when life suddenly changes drastically.

Sometimes, parents also tend to put too much pressure on themselves to be perfect mothers and fathers, exacerbating mental health conditions.

"Both mothers and fathers need to pay attention to their mental health during the perinatal period, and they need to watch for these other types of conditions, not just depression," says Wendel-Hummell, a PhD in sociology whose research focus brings together biological and sociological understandings of the problems that new parents face. "Anxiety, post-traumatic stress disorder, psychosis and bipolar disorder are all shaped by circumstances that surround having a baby."

Kenneth Lim, 34, father of a 3½-year-old and 16-month-old, reckons that a father's health "tends to decline a little in the first five years of your child's life" until a tipping point is reached. "Then you either decide to start working out again, or just let yourself go and have the dad bod," he says.

It's not all bad news for new dads though; research shows fatherhood does have its health merits. First-time fathers show significant decreases in negative lifestyle behaviours such as tobacco and alcohol use, as well as crime, according to a 2011 study in the Journal of Marriage and Family that assessed 200 at-risk boys annually from age 12 to 31.

Fatherhood has probably been the most significant bout of prolonged exhaustion I've experienced

Anand Chandran, investment banker

"These decreases were in addition to the general tendency of boys to engage less in these types of behaviours as they approach and enter adulthood," says the study's lead author, David Kerr, an assistant professor of psychology at Oregon State University.

"This research suggests that fatherhood can be a transformative experience, even for men engaging in high-risk behaviour. This presents a unique window of opportunity for intervention, because new fathers might be especially willing and ready to hear a more positive message and make behavioural changes."

Another study in 2011 suggested that fathers have a 17 per cent lower risk of dying from cardiovascular disease than childless men. Stanford University arrived at this finding after tracking 135,000 men aged 50 to 71 over a 10-year period.

Also declining among new dads are testosterone levels, according to a University of Michigan study last year. The hormone-lowering process starts even before the babies are born. Fathers' testosterone levels have been linked with parenting behaviour and involvement, with higher levels associated with lower parental involvement.

A fully involved father was what Anand Chandran wanted to be when his firstborn arrived 16 months ago. In the initial months after Leah's birth, the 32-year-old investment banker almost stopped his regular workout regime as he wanted to be around his wife and daughter.

"After a while, I realised I needed to build a routine into my life if I was going to stay fit. So I sorted out my priorities, with fitness being number two behind daddy and husband duties," he says.

Chandran rises well before sunrise to cycle or run, and returns in time to see his family wake up. This resolve has made his #dadbod an anomaly; a fitter and buffer upgrade from his pre-dad bod.

"Fatherhood has been tiring - probably the most significant bout of prolonged exhaustion [I've experienced]," says Chandran. "It's tough trying to fit other priorities in but having at least one other pursuit like exercise in the weekly routine helps. It alleviates a bit of the stress and fatigue of parenting with the endorphin boost, and makes sure you're defined as a little more than just dad or husband. Also, I want my daughter to lead a healthy and active life, so I need to start living the routine myself."